A single buyer has snapped up more than half the copper held in London Metal Exchange warehouses, giving it control over a crucial source of supply and raising concerns among traders about the potential for higher prices.

On several occasions in the last month, this buyer held as much as 90% of the world’s copper stored in LME-licensed warehouses, equal to about 140,000 tons, or enough to make the copper parts of the Statue of Liberty more than 1,700 times. As of Wednesday, the buyer owned between 50% and 80% of copper held...